In membrane separations, each membrane has the ability to transport one component more readily than the other because of differences in physical and chemical properties between the membrane and the permeating components. Furthermore, some components can freely permeate through the membrane, while others will be retained. The stream containing the components that permeate through the membrane is called permeate and the stream containing the retained components is called retentate. The transport of permeate across the membrane is achieved by the application of either mechanical, chemical, electrical or thermal works.

Membrane separation process has emerged as a separation technology that undergoes a rapid growth during the past few decades. It has drawn the world attention especially in the separation technology field, which is competitive in many ways with conventional separation techniques such as distillation, adsorption, absorption and extraction.

Practically all filtration membrane is based on synthetic organic (polymeric or liquid) or inorganic (ceramic, metal) membrane. But for the sake of simplicity only the synthetic organic polymer membranes are discussed here. A synthetic organic polymer can be classified according to different viewpoints such as morphology or separation mechanism.

In term of morphology, two types of membrane were distinguished: symmetric or asymmetric. In the symmetric membranes, the diameter of the pores is almost constant throughout the cross section of the membrane. Furthermore, the entire membrane thickness causes resistance to mass transfer acting as a selective barrier. In asymmetric membrane, the pore size at the surface have a different size compared with the pores at the bottom side. Large particles will not enter in the body of the membrane. In this way, the plugging of the membrane is avoided. It also possible that the top layer is non-porous or even made of a different material and such membrane is called composite membrane. The composite membrane composed of two layers, a support layer and a skin layer. The support or porous layer has high porosity, no selectivity and a thickness of 50 to 150 mm. The skin or top layer is very thin (0.1 to 5 mm) and it is responsible for the membrane selectivity. The actual separation mechanism can be based on following:

i. Separation by size – the sieve effect.

This requires porous membrane with rather large pores. These mechanisms are the simplest form regarding mode of separation. Terms like macropores, mesopores and micropores are used to describe the pore sizes in membrane for microfiltration, ultrafiltration and nanofiltration. These membranes have been designed to retain larger solute and suspended particles thus remove contaminants based on size by sieving mechanism.

ii. Separation by different in solubility and diffusive of materials.

The separation mechanism based on diffusion of the solute and solvent through the membrane. These membranes are often in the form of composite of a homogenous film on a microporous support as used in reverse osmosis pervaporation. Both of these membranes has the smallest pores and allows only the solvent to pass through by sorption-diffusion mechanism.

iii. Separation by charge.

An ion-exchange membrane separates compounds of different charges such as separation of ions from water and non-ionic solutes. These membranes carry either fixed positive or negative charges or separates by exclusion of ions of the same charge as carried in the membrane phase. Ion-exchange membranes are generally used in operation like electrodialysis and major application is as electrolytic cell separators in for example the production of chlorine and caustic soda. Table 1 provides an overview of the various types of membrane process.

Basic Principle of Membrane Separation

The performance of a membrane can be distinct by two simple factors, flux (or product rate) and selectivity through the membrane. Flux is defined as the permeation capacity that refers to the quantity of fluid permeating per unit area of membrane per unit time. Flux depends linearity on both the permeability and the driving force. The flux also depends inversely upon the thickness of the membrane. Thinner membranes contributed to the higher flux. Usually the water flux is measured in gallons per square foot per day (galft-2day-1), kilogram per square meter per hour (kgm-2 hr-1) or meter cube per meter square per day (m3m-2day-1). Membrane selectivity towards a mixture is generally expressed by rejection. It is a measure of the relative permeation rates of different components through the membrane. The simplest manner to express solute rejection characteristics is defined as:

R =1 – Cp / Cf

where Cp and Cf denote concentration of permeate and concentration of feed/bulk solution respectively, and both can be measured. Ideally a membrane with high selectivity or rejection and with high flux is required, although it was observed that an attempt to maximize one factor is compromised by a reduction in the other.

Table 1: Overview of membrane separation process, their operating principles and applications

Membrane Separation in Industry

Desalination of sea or brackish water entails forcing salt solution through a permselective membrane at pressure, which is sufficiently high to overcome the osmotic forces and tends to drive water in the opposite direction. These membranes must allow water to permeate at high rate but must reject permeation of the salt molecules to a high degree.

A breakthrough of membrane to industrial applications begun in 1960 with the invention of the first integrally skinned asymmetric cellulose acetate hyperfiltration membrane by Loeb and Sourirajan. This membrane consist of a very dense top layer or skin with thickness of 0.1 to 0.5µm supported by a porous sub layer with a thickness of about 50 to 150µm. These membranes combine the high selectivity of a dense membrane with the high permeation of a very thin membrane.

In 1970’s, the first commercial composite reverse osmosis membrane was developed. The membrane consists of a very thin dense top layer, which is supported by a porous sub layer of a different material, which is quite different from the asymmetric cellulose acetate membrane where it is developed with two layers of the same material. The advantage of the so-called thin film composite (TFC) membrane is that each layer can be optimized independently to obtain optimal membrane performance with respect to selectivity, permeation rate and chemical and thermal stability.

Membrane separation is still evolving and finding more and more applications in a broad range of fields and the development of membrane will strongly influence separation process in the future. The main force of membrane separation is the fact that it works without the addition of chemicals, with a relatively low energy use and easy and well-arranged process conductions. Rapid growth in membrane separation development is primarily based on consciousness on the potential of this technology. Nowadays it is widely used in many applications like industrial wastewater treatment, desalination of sea and brackish water and liquid food treatment.